1. Field of the Invention
This invention relates to a male and female screw rotor mechanism for use in screw compressors or the like, and more particularly to improvements in screw rotors of the type which consists of a female rotor with an addendum on the outer side of a pitch circle of its respective teeth and a male rotor having a deddendum on the inner side of a pitch circle of its respective teeth correspondingly to the addendum of the female rotor.
2. Description of the Prior Art
A screw compressor was originally invented by Krigar in Germany in about 1878 and ever since various improvements have been made in this connection. In place of the so-called symmetrically toothed rotors which were used in the original screw compressor, SRM (Svenska Rotor Maskiner Aktiebolag) of Sweden introduced in 1965 asymmetrically toothed rotors with markedly improved volumetric efficiency. An example of the asymmetically toothed rotors can be seen, for example, in Japanese Patent Publication No. 56-17559 which discloses rotors of the construction as schematically shown in FIG. 1.
In this case, it is intended to increase the theoretical volume by forming an adendum Af on the outer side of a pitch circle Pf of each tooth of a female rotor F and forming a corresponding deddendum Dm on the inner side of a pitch circle Pm at each root of a male rotor M, shaping the teeth of the female and male rotors in the shapes with the following characteristics.
(1) Female Rotor Tooth Shape PA0 (2) Male Rotor Tooth Shape
(a) Tooth shape on the leading side
Profile n-d is formed by an arc having its center at the intersection of the pitch circle Pf and a line drawn through the centers (or axes) Of and Om of the female and male rotors, and &lt;nmd is about 10 degrees. Point n is located on the interaxial line Of-Om.
Profile d-e is formed by an arc having its center at point k on an extension line of radius d-m. Point e is located on the pitch circle Pf.
(b) Tooth shape on the follower side
Profile n-c is an arc having its center at point m, and &lt;nmd is about 10 degrees. Accordingly, &lt;cmd is an arc of about 20 degrees.
Profile c-a is a generating curve which is determined by point h of the male rotor.
Profile b-a is an extension line of a straight line Of-b. Point a is located on the pitch circle Pf.
(a) Tooth shape on the leading side
Profile p-i is an arc having its center at the intersection of the pitch circle Pm and a straight line drawn through the centers Of and Om, and conforming with the arc n-d of the female rotor. Point p is located on the inter-axial line Of-Om of the rotors.
Profile i-j is a generating curve which is determined by the arc d-e of the female rotor. Point j is located on the pitch circle Pm.
(b) Tooth shape on the follower side
Profile p-h is an arc having its center at point m and conforms with the arc n-c of the female rotor.
Profile h-g is a generating curve which is determined by point b of the female rotor.
Profile g-f is a generating curve which is determined by a straight line b-a of the female rotor. Point f is located on the pitch circle Pm.
The present invention contemplates further improvement of the volumetric efficiency in screw rotors of this sort (which is about 83.99% in the particular example given above). It has been known in the art that the volumetric efficiency is largely influenced by the following three factors: the theoretical volume; the seal line length per unit theoretical volume; and the blow hole area per unit theoretical volume.
With regard to the theoretical volume, under the restrictions imposed by the predetermined distance CD (Om-Of) between the centers of the male and female rotors, an arrangement should be made in such a manner as to increase the theoretical volume to a maximum, namely, to increase the outer diameters of the male and female rotors so as to be as large as possible. However, this problem cannot be solved simply by increasing the outer diameters of the male and female rotors M and F. This is because mere enlargement of the outer diameters of the male and female rotors will result in a reduction in the tooth width of the female rotor F and hence in a material reduction in mechanical strength. This problem arises conspicuously particularly in the case of rotors with the conventional tooth shapes as shown in FIG. 1.
More specifically, as mentioned hereinbefore, the generating curve c-d in the conventional tooth shape of FIG. 1 is formed by point h, and partly located to the follower side by the angle &lt;hn. Therefore, if the outer diameters of the rotors were increased, the female rotor would be largely scooped or recessed along the generating curve c-b, as a result reducing the tooth width of the female rotor. It follows that, in order to enhance the volumetric efficiency of the screw rotors of FIG. 1, it is necessary to define tooth shapes which will permit an increase in the outer diameters of the male and female rotors without an accompanying material reduction in the tooth width of the female rotor.
In addition, there is another problem which will arise as a result of mere enlargement of outer diameters of the male and female rotors, i.e., a problem concerning the seal line length and blow hole area. That is to say, mere enlargement of the outer diameters of the male and female rotors will invite increases in the seal line length and the blow hole area, lowering the volumetric efficiency to the contrary.